1. Field of the Invention
The present invention relates to a thin film transistor and fabricating method thereof, further, a semiconductor device and a manufacturing method thereof.
2. Description of the Related Arts
In recent years, researches on high performance semiconductor devices (specifically, liquid crystal display devices, EL display devices and other display devices) using a thin film transistor have been promoted. A thin film transistor (hereinafter, referred to as a TFT) having high mobility needs to be realized particularly for a semiconductor device which is required to have high-speed performance and increased functionality. As a method for improving the crystallinity of the semiconductor film, a metal element, as typified by Ni, which promotes crystallization of an amorphous silicon film is doped, formed into a film, or applied on the semiconductor film (all of which are referred to as forming a substance having a metal element), and then, the semiconductor film is heated and a crystallization process for forming a crystalline semiconductor film is performed on the semiconductor film.
In the crystallization process described above, a crystalline semiconductor film with large-size particles is obtained by using a metal element typified by Ni to promote crystallization. In addition, the crystalline semiconductor film that grain boundaries are connected with each other at high ratio and grain defects are less is obtained.
However, a metal element such as Ni has an adverse effect on device properties (electric properties) of a TFT, and therefore, a step for removing the elements (hereinafter referred to as a gettering) is carried out. A gettering is known as a technique for segregating metal impurities brought in a semiconductor to a gettering sink (or it is also called a gettering site) with some kind of energy, thereby reducing impurity density in an active device region.
Gettering processes are divided broadly into two categories, an extrinsic gettering and an intrinsic gettering. The extrinsic gettering provides a gettering effect by applying a strain field or chemical action from the exterior. Meanwhile, the intrinsic gettering utilizes a strain field of a lattice defect caused by oxygen that is generated inside a single crystal silicon wafer.
As a specific example of gettering processes, a gettering is performed by forming a region added with a noble gas element (noble gas) or a semiconductor film added with a noble gas element and moving a metal element thereinto by heat treatment. Thereafter is removed the region doped with a noble gas element (noble gas) or the semiconductor film doped with a noble gas element (Reference: Japanese Patent Laid-Open No. 2002-313811).
However, when a gettering sink is removed by gettering according to the above-described method, the gettering sink could not be etched rightly in some cases. That is, even when a gettering sink is removed by using alkaline solution of etchant having a high selectivity to the gettering sink and a barrier film functioning as an etching stopper, residue of the gettering sink is left.
A TFT having such residue of gettering sink has a negative characteristic such as bad leakage of off-current (Ioff) and low reliability. Further, defects generate in a mass-production line due to the gettering residue, and thus, the yield ratio decreases.